1. Field of the Invention
The present invention relates to electro-optical modulators. In particular, the present invention relates to a method of analog modulation of an optical carrier signal according to an electrical modulating signal, and to an optical emitter which uses this method.
2. Description of the Related Art
An optical carrier signal can be modulated directly, by acting on the optical source, usually a laser, or indirectly, by means of an external optical modulator connected to the output of the optical source.
An example of an optical modulator which can be used for the amplitude modulation of an optical carrier signal with a modulating signal having a high frequency is an interferometer of the Mach-Zehnder type, constructed on lithium niobate (LiNbO3).
A required characteristic of the external modulator is linearity of modulation, in other words the modulator must not distort the transmitted information.
The electro-optical characteristic (optical power as a function of the radio-frequency voltage) of modulators of the indicated type (the Mach-Zehnder interferometer) is typically non-linear. To limit the signal distortion, it is helpful to make the modulator operate in the vicinity of the portion of its characteristic that is as nearly linear as possible.
For this purpose, a modulating signal at radio frequency (RF) is applied to an RF port of an electro-optical modulator of the Mach-Zehnder interferometer type, and a direct-current voltage, in other words a bias signal, which determines the operating point, or bias point, of the modulator, is applied to the same port or to a suitable BIAS port.
An example of a modulator of this type is that marketed by the applicant under the symbol PIR PIM1510.
The modulating signal, applied to the RF input, consists, for example, of a sequence of bits of suitable amplitude.
In the case of a Mach-Zehnder modulator, the shape of the characteristic about to a sinusoid, and it is advantageous for the modulator to operate in the vicinity of the inflection point of the sinusoid when the operating point voltage VQ is applied.
The modulation characteristic of the Mach-Zehnder modulator, related to the operating point, can be expressed by the relation:Pu=Kz sin β
where:                Pu is the output optical power;        Kz is a coefficient which depends on the characteristic of the Mach-Zehnder modulator;        β=πV/Vπ is the modulation index of the modulating signals, expressed in radians;        V is the variation of the applied voltage with respect to the operating voltage VQ;        Vπ is a constant.        
This characteristic with the sinusoidal shape is identified by two values:                the value of the voltage, called Vπ, which represents the variation of voltage to be applied to the RF electrode to bring the output optical power from the maximum to the minimum value;        the value of the voltage VQ which has to be applied to the power supply electrode to make the operating point correspond to the inflection point of the characteristic with the sinusoidal shape, in other words with odd symmetry. In this case, the distortions of even order (including the second harmonic of the applied signals) and the distortions of odd order assume well-defined values.        
For example, in the case of a Mach-Zehnder modulator of the type PIR PIM1510, produced by the applicant, the aforesaid voltages may assume the following values: Vπ=4.3 V and VQ=0.7 V.
The value of the voltage VQ of the operating point is not constant, but varies with time (as a result of the accumulation of static charges in the LiNbO3, for example) and also with temperature.
Consequently, the operating voltage must be continuously adapted, on the basis of information such as the presence and size of the distortions of even order, in other words the intermodulation products of the second order.
Patent application EP 768 765 describes a method of controlling the operating point, or bias point, of an electro-optical modulator for CATV (community antenna television) systems. According to this method, an additional signal, called the pilot tone, having a frequency lower than the lower limit of the modulation signal band, is injected into the electro-optical modulator. The second harmonic of this pilot tone is detected at the output of the electro-optical modulator, and a bias signal is generated according to the sign and amplitude of this harmonic.
In the article “Quasi-feed-forward linearization of electro-optic modulators for analog signal transmission” by D. Davidson et al., Optical Engineering, vol. 32, No. 4, Apr. 1993, the authors describe a linearization, known as a “quasi-feed-forward” linearization, used for electro-optical modulators of the Mach-Zehnder type, used in the analog modulation of optical signals for CATV systems.
The linearization system described in the article requires the use of two Mach-Zehnder modulators, EOM1 and EOM2. A fraction of a radio-frequency signal is applied to the modulator EOM1. The optical signal leaving the modulator EOM1 is applied, after conversion to an electrical signal and a 180° phase shift, to the modulator EOM2. Another fraction of the radio-frequency signal is applied directly to the modulator EOM2. The electrical signal applied to the modulator EOM2 consists of the original radio-frequency signal plus the distortion products generated by the modulator EOM1, phase-shifted by 180°. This system substantially pre-distorts the radio-frequency electrical signal to be injected into the modulator EOM2, using an electro-optical modulator EOM1.
In general, in WDM (wavelength division multiplexing) optical transmission systems, Mach-Zehnder modulators are used to modulate an optical carrier having a wavelength contained in a low-attenuation window of the optical fibres with a modulating signal with which is associated the information to be transmitted.
Patent application EP 98117898.1 describes an optical transmission system for 128 channels, comprising 128 wavelength converters.
Each of these wavelength converters comprises a photodiode for receiving an optical signal generated by an external source, and for converting it to an electrical signal; a laser for generating a fixed-wavelength optical carrier; and an electro-optical modulator, such as a Mach-Zehnder modulator, for modulating this carrier with the said electrical signal. These converters can be used to convert an optical signal having a certain wavelength to a signal having a predetermined wavelength and then to re-transmit it.
In the field of communications, and particularly in the field of optical communications, there are various known information transmission protocols, such as SONET (Synchronous Optical Network), ATM (Asynchronous Transfer Mode), SDH (Synchronous Digital Hierarchy), and IP (Internet Protocol).
For example, in a digital optical communications system using the SDH-STM-16 protocol, the data signal requires a sequence of binary digits (bits) having a bit rate rb=2.488 Gbit/s, generated by time division multiplexing of further sequences of binary digits at lower frequency. This sequence is associated with code words which represent the information, and have a length M (number of bits contained in the word).
Additionally, this signal is obtained by means of a masking, or “scrambling”, operation, which makes it possible to create a sequence of bits having a spectral content distributed uniformly along the frequency axis, in other words without clustering in particular bands, in which the probability that there will be bits equal to 1 is equal to the probability that there will be bits equal to 0.
The spectrum of this signal is discrete and has spectral lines spaced apart by the frequency of repetition of the word, in other words by the distance rb/M.
Standards ITU-T-925 and ITU-T-803 (International Telecommunications Union) contain, respectively, the standards for the SDH-STM-16 protocol and those for the corresponding optical signal.
The word which has actually been transmitted is discriminated from the set of specified possible words at a suitable receiver present in the transmission system.
Because of various noise factors, there is a probability of error in the receiver, in other words the probability of incorrect discrimination of the transmitted word may be significant.
A method used in optical communication systems to minimize this probability of error is that called “forward error correction” (FEC), according to which additional data, containing codes for identifying and correcting the error, are associated with the signal to be transmitted. For example, one error identification bit is inserted for about every 16 bits of the data signal.
At the point of reception, a suitable FEC decoder identifies the errors which have occurred during transmission and corrects them.
The introduction of the error codes results in an increase of the bit rate rb: for example, for an SDH signal having a bit rate rb=2.488 Gb/s, the bit rate is changed to rbFEC=2.666 Gb/s.
For generating an SDH signal with FEC, a further scrambling operation is carried out according to an FEC code word having a length of MFEC, equal to 213−1 for example.
The spectrum of the SDH signal with FEC has lines spaced apart by a quantity equal to rbFEC/M MFEC.